When proteins useful as pharmaceuticals are produced with the recombinant DNA technique, use of animal cells enables complicated post-translational modification and folding which prokaryotic cells can not perform. Therefore, animal cells are frequently used as host cells for producing recombinant proteins.
Recently, a large number of biopharmaceuticals, such as antibodies and physiologically active proteins, have been developed. Techniques that permit efficient production of recombinant proteins by animal cells lead to cost reduction of biopharmaceuticals and promise their stable supply to patients.
Under these circumstances, a method of protein production with higher production efficiency is desired.
It has been known that the number of copies of dihydrofolate reductase (DHFR) gene is amplified (gene amplification) in cells by methotrexate (MTX), whereby the cells become MTX-resistant. Widely used in industrial production is a method for increasing the amount of useful protein production, in which plasmids having a gene of the protein connected downstream of the DHFR gene are introduced into animal cells which are then cultured in an MTX-supplemented medium to induce gene amplification (Patent Document 1).
[Patent Document 1]
    Japanese Patent Publication No. 6-30588